This invention relates generally to lifting devices and, in particular, to load supporting and lifting forks for use in such lifting devices.
More specifically, this invention relates to forks suitable for use in forklift trucks of the type generally used to lift and transport flat loads or load bearing pallets. Most forklift trucks employ a pair of L-shaped forks that are mounted in spaced-apart alignment upon a lifting frame. The vertical leg of each fork is attached to the frame, while the horizontal legs project forwardly from the frame. The frame is adapted to move the forks up and down in a vertical direction so that the horizontal legs can be lowered for insertion beneath a load and then raised to lift the load. In addition, most lifting frames can be tilted rearwardly toward the truck thereby canting the load and thus preventing it from sliding forwardly on the forks.
Although a load situated between the canted forks of a lift is prevented from sliding longitudinally along the forks, it has been found that the load is still capable of sliding laterally thereby either dangerously unbalancing the load or allowing it to fall from the lift. In either case, the shifting load poses a danger to both workers and equipment. The load contacting surfaces of most forks are made of flat, high grade, steel which takes on a highly smooth, polished texture with use, thereby increasing the dangers associated with lateral load slippage.
Padgett, U.S. Pat. No. 3,777,923 discloses a pallet transport that has a pair of horizontally-extended lifting arms which can be inserted under a load bearing pallet. The arms are rotated upwardly about a pair of rear wheels to raise the pallet. Each arm is equipped with a longitudinal bar having sharp raised teeth cut therein. The teeth are designed to bite or dig into the bottom of the pallet to prevent it, and thus the load, from slipping when in transit. The teeth, although providing for the safety of the load, never-the-less cause considerable damage to the pallet each time the pallet is lifted. This type of device, therefore, is completely unsuited for lifting such things as stacked lumber, or the like, where the teeth come into direct harmful contact with the load.
Watkins in U.S. Pat. No. 2,659,506 describes a fork truck that is equipped with a pair of L-shaped lifting forks. Watkins recognizes that the load contacting surfaces of the forks do not provide sufficient frictional holding power to prevent even heavy loads from slipping or sliding thereon. Watkins provides the contact surfaces of his forks with a series of "grab blocks" which are carried in recesses formed in the forks. Each grab block contains a number of V-shaped ribs that are arranged to contact a load situated on the forks. Here again, in order to effectively prevent slippage, the ribs must dig into the load and thus can do harm to the load. Furthermore, the blocks and the recesses formed in the forks must be accurately machined to accept and securely hold the block. This considerably increases the overall cost of the lifting device.